The so-called Roswell Incident of 1947 spawned conspiracy theories by the score.

But now, sadly for UFO spotters, a new book offers an entirely man-made – and some would say even more bizarre – explanation, featuring two of the greatest villains of 20th century history: the Soviet dictator Josef Stalin and the infamous Nazi “Angel of Death” Dr Joseph Mengele.

During a powerful storm in July, 1947, an object crashed near a rancher’s home in Roswell, New Mexico.

Roswell army air base initially said that a “flying disc” had come down, but hours later, as government scientists arrived in the area, it was stated instead that a weather balloon had crashed.

The incident went largely unreported until books and documentaries in the 1970s made allegations of alien life forms.

Area 51, the new book by Annie Jacobsen, is based on interviews with scientists and engineers who worked in Area 51, the top secret test base in the Nevada desert.

It dismisses the alien story and puts forward the theory that Stalin was inspired by Orson Wells’s famous radio adaptation of the HG Wells novel War of the Worlds, which provoked hysteria across America when broadcast in 1938. According to the book, the plot started after the Soviet Union seized from Germany at the end of the war the jet-propelled, single wing Horton Ho 229 – a fighter said to be the forerunner of the modern B2 stealth bomber.

This is where Mengele enters the story. The Nazi doctor, who experimented on prisoners in Auschwitz and fled to South America after the war, was supposedly enlisted to create a crew of “grotesque, child-size aviators” in return for a eugenics laboratory.

The book says that the plane was filled with “alien-like” children, aged 12 or 13, who Stalin wanted to land in America and cause hysteria similar to the 1938 broadcast. But, the plane, remotely piloted by another aircraft, crashed and the Americans hushed up the incident.

Jacobsen’s source, a retired engineer from the former defence company EG&G, which handled the US government’s most sensitive projects, said he was put on to the Roswell project in Area 51 in 1978.

Miss Jacobsen writes: “They found bodies alongside the crashed craft. These were not aliens. Nor were they consenting airmen. They were human guinea pigs. Unusually petite for pilots, they appeared to be children. Each was under five feet tall.

“They were grotesquely deformed, but each in the same manner as the others. They had unusually large heads and abnormally shaped oversize eyes.”

Two were supposedly found “comatose but still alive”.

The EG&G engineers were told “that seeing the bodies would be a shocking and disturbing experience”.

Perhaps not surprisingly, a spokesman for the US Air Force said: “We have not yet read this book so we are not able to make a comment on it.”